Niemann-Pick Type C (NPC) disease is a lethal inherited neurovisceral lysosomal storage disorder caused primarily by recessive mutations in the NPC1 gene. NPC disease is characterized physiologically in humans, and in animals broadly, by defects in cellular cholesterol homeostasis leading to defects in autophagy. Specifically, unesterified cholesterol derived from the low-density lipoprotein (LDL) pathway is taken up by cells but unable to be assimilated metabolically because cholesterol-laden compartments are unable to fuse with lysosomes, resulting in reduced catabolism and a state of cholesterol limitation. Some cell types and tissues are hypersensitive to cholesterol limitation or disruption of autophagic flux, as well as to secondary accumulations of other cellular lipids such as sphingomyelin. Additional pathophysiologic complexity arises because cholesterol is the precursor for essential metabolites that include steroid hormones, oxysterols, bile acids and cell membrane itself. Brain, liver, spleen and lung are among the organs that manifest pathophysiology in NPC patients. Although clinical presentation and time of disease onset varies from patient to patient, NPC disease ultimately leads to premature death in young adulthood by a progressive neurodegenerative decline.
Multiple animal models of NPC disease are possible because the NPC1 gene is evolutionarily ancient and conserved throughout the Animal Kingdom. The genomes of baker's yeast (Saccharomyces cerevisiae), nematodes (Caenorhabditis elegans), fruit flies (Drosophila melanogaster), zebrafish (Danio rerio) and mice (Mus musculus) all have an ancestral version of the NPC1 gene. Cholesterol limitation is observed across multiple species. NPC disease models have been generated and studied in nematodes, fruit flies, zebrafish and mice. Nematodes lacking the NPC1 gene are hypersensitive to cholesterol restriction, fail to reach adulthood with normal timing, and produce smaller broods because specific cells have defects in autophagic flux, resulting in the inability to convert cholesterol to an essential hormone called dafachronic acid. Similarly, fruit flies lacking the NPC1 gene usually die as early-stage larvae due to the inability of specific cells to convert cholesterol to an essential hormone called ecdysone. Fruit flies that survive into adulthood exhibit neurodegeneration and other disease-relevant phenotypes. Mice lacking the NPC1 gene die prematurely due to neurological decline and rapid weight loss, with multiple cell types exhibiting phenotypes of cholesterol limitation.
There is no FDA approved treatment for NPC. Current experimental drugs for NPC are being repurposed and were discovered on the basis of normalization of a cellular phenotype in patient-derived fibroblasts, namely clearance of LDL-derived cholesterol storage. However, cells derived from NPC patients with certain mutations do not exhibit the cholesterol storage phenotype, demonstrating that while this storage phenotype is diagnostic of disease it is not the means to reversing disease. The compounds described herein were discovered first in a whole animal, then validated in a patient cell model, and then further validated in a mouse model, resulting in compounds that reverse the root causes of disease via a bypass pathway rather than by reversing a cellular phenotype that may simply be a feature of disease.